The development of combined de-NOx and de-SOx processes has been a subject of intensive research worldwide since the 1980's. Recently, Ameur (2001) conducted a literature review and developed a database on combined de-NOx and de-SOx processes, and concluded that most of the so-alled “combined” processes developed are not “simultaneous” but rather “successive” processes, combining one process for each family of pollutants. This often makes the addition of pollution abateman equipment to existing units too costly because of the need for two separated processes. Moreover, most of these processes require extensive modification to existing facilities, which is not always feasible, especially for smaller facilities.
Existing de-NOx and de-SOx processes may be classified into three groups, namely, catalytic, non-catalytic/chemical, and electronic.
The catalytic processes often involve the use of expensive catalysts, in which NOx is converted to nitrogen gas via selective catalytic reduction (SCR), while SO2 is oxidized to sulphur trioxide and absorbed to form sulphate/sulphuric acid. Catalysts may be poisoned, often by SO2, and need to be regenerated. These processes function, in many cases, as a reaction-regeneration cycle. Current development in this area is mainly aimed at new catalysts with long lifetime and high activity. Examples of elements and metals commonly in oxide form tested include, copper, cerium, vanadium, titanium and the like.
Most of non-catalytic/chemical technologies are based on the fact that both NOx and SOx are acidic oxides and wherein various alkaline materials are used to remove NOx and SOx. The widely used lime/limestone processes belong to this group. Waste disposal and low efficiency are, inter alia common problems of this group. Non-catalytic/chemical removal can be carried out in either dry and wet modes, wherein while the latter often enjoys higher removal efficiency, it faces a bigger disposal problem. Fluidized bed combustion (FBC) technology may be considered a member of the dry group. Since the removal of NOx and SOx is carried out during combustion in a fluidized bed, it is not a post-combustion technology.
Electron-based technologies use various forms of electric energy such as corona, plasma, and electron beam to enhance removal. In plasma technology, for example, an ultra-high voltage is utilized to produce radicals with addition of a chemical, such as ammonia. Non-thermal plasma, however, offers a high-energy efficiency, since energy is directed into increasing electron motion in chemical species not heating (Hackman and Akiyama, 2000). These technologies tend to be capital-extensive.
Two main types of petroleum coke are produced in the “so-called” upgrading process, namely, delayed coke and fluid/flexi coke. In view of only minor differences between fluid coke and flexi coke, they are often grouped together under the name of “fluid coke”. Due to the difference in the production technology employed, particularly in process temperature, a typical fluid coke has a lower volatiles content, a higher bulk density, higher sulphur (ca 8%) and ash content than a typical delayed coke. Physically, fluid coke comprises particles having a particle size of about 200 μm in diameter with an onion-like layered structure, while delayed coke is produced in the form of large lumps.
Worldwide production of delayed coke exceeds fluid coke by several times. In Western Canada, daily production of oil sands fluid coke exceeds 6000 tonnes. Mainly due to its high sulphur content, almost all fluid coke produced is being landfilled and added to the existing stockpiles of about 45 million tonnes. It has been suggested that the fluid/flexi coking is “a front runner” among technologies for upgrading heavy crude to transportation fuel (Furimsky, 2000). Moreover, it was predicted that the production of petroleum coke would increase as a result of the increased amount of lower-quality high-sulphur crude oils treated (Swain, 1997). An increase in sulphur in coke is also anticipated, as more sulphur has to be rejected to meet the increasingly strict regulations on sulphur in transportation fuel. Clearly, there is a need to develop new, preferably, beneficial uses for high-sulphur petroleum coke, particularly fluid coke.
As is well known in the field, fluid coke produced at high temperature is refractory, that is, it has a graphitic or glassy surface and is unreactive. For example, this type of unreactive coke is produced when sulphur containing coal or oil is pyrolysed to produce volatile gases as fuels and residual refractory coke. The sulphur containing refractory coke has limited use from two aspects. Although the coke can be combusted as a fuel, upon combustion, the sulphur is converted to waste gas-containing sulphur dioxide which gas must be treated to prevent release of the sulphur dioxide to the environment. The second reason is that the refractory coke has low surface area and, hence, cannot be effectively used as an activated carbon for absorption or catalytic purposes. Both of these issues are addressed in the present invention.
Sulphur content is at the centre of the challenges to using petroleum coke and invariably determines the end market for the coke. Low sulphur coke (<2 wt %) is often used for the production of anodes and other high value products, while a coke with 2 to 5 wt % S is considered to be fuel grade. Although fluid coke constitutes a significant energy source having very high heating values (32-35 MJ/kg), its utilization as a solid fuel in conventional pulverized coal (PC) burners is limited and, more often, prevented by the heavy burden added on traditional lime/limestone-based flue gas desulphurization (FGD).
According to Anthony (1995), it was concluded that “fluidized bed combustion (FBC) is the best, and only available technology for burning alternative fuels” with elevated sulphur levels, such as petroleum coke. In a FBC boiler, sulphur and nitrogen are captured during combustion and become part of the ash produced. However, FBC is not a de-NOx and de-SOx technology for treating flue gas. Limestone is often added at a typical Ca/S ratio of 2 to capture sulphur. Despite the reported high efficiency, sulphur capture in FBC remains one of the key issues in improving economics of the technology. Other limitations identified with FBC include fireside fouling that is closely linked to high sulphur in fuel (Anthony and Jia, 2000). Further, ash production and disposal problems are related to sulphur content. Desulphurization prior to utilization has also been studied. Unlike coal, sulphur in coke is largely organic in nature. Mechanisms of desulphurization, therefore, involve the cleavage of C—S bonds. In 1970's, Tollefson's group in Calgary pioneered the desulphurization of coke using hydrogen, and later improved the efficiency with ground coke particles and NaOH. It was found that fluid coke was more resistant to desulphurization than delayed coke. Molten caustic leaching, which was originally developed for removing organic sulphur in coal, was applied to both fluid and delayed coke at 200 to 400 C, resulting in less than 1% of sulphur (Ityokumbul, 1994). It was found, however, that no process developed so far had proven to be economically viable. Recently, Furimsky (1999) suggested that gasification could emerge as another alternative for utilizing petroleum residues, including coke. A group at Tohuku University investigated gasification with various metal hydroxide catalysts (Yamauchi et al., 1999). In two separate studies carried out in England and Spain, petroleum coke was added to a typical industrial coal blend used in the production of metallurgical coke (Barriocanal et al., 1995; Alvarez et al., 1998). At the University of Alabama, petroleum coke was tested for remediating Sucarnoochee soil (15 wt % oil) via a two-step agglomeration process (Prasad et al., 1999). Under the optimal condition, the remaining oil content in the soil was found to be below 200 ppm.
Production of activated carbon (AC) from petroleum coke was investigated mostly on delay coke by several groups. From a mixture of various kinds of petroleum coke with an excess amount of KOH, Otowa et al. (1997) obtained an AC maximum surface area of 3000 m2g−1 at 600 to 900 C. The product is commercially available under the trade-mark of MAXSORB™. The process for producing this material is described in U.S. Pat. No. 5,401,472 Mar. 28, 1995 In claim 1 ‘An apparatus for the production of active carbon through activation of a carbonaceous material with an alkali metal hydroxide . . . ” is described. The activated coke product is a low sulphur (ie 86 ppm S in Example 1 of patent) and the process clearly does not address the activation of a high sulphur content delayed coke material such as the present invention describes.
A delayed coke having 7% wt % S was activated with NaOH and KOH at 400 to 600 C by Lee and Choi (2000), to proceed AC with a surface area of 977-1350 m2g−1. They reported that the surface area did not increase substantially until the residual sulphur was reduced to below 0.1% S. Thus this process would not be suitable for a high sulphur coke unless the sulphur level was first reduced by leaching.
Zamora et al (2000) used H3PO4, NaOH and ZnCl2 to activate a petroleum coke, likely fluid coke, having a high sulphur content (6 wt %), to obtain an AC BET surface area of about 16-35 m2g−1, with H3PO4 being the most effective agent. The use of phosphoric acid would cause environmental problems due to restrictions on disposal of phosphates and the degree of surface area development is not sufficient for most industrial applications.
A study on activating Syncrude fluid coke was conducted by DiPanfilo and Egiebor (1996). Using steam at 850 C for 6 hours, a maximum surface area of 318 m2g−1 was obtained. Pretreatment of KOH was found to increase activation rates, but not surface area. Their data revealed a significant decrease in sulphur content after activation. As expected, the exhaust gas from the steam activation process had a high H2S level (ca 5% V) which cannot be released to the environment and would have to be removed from the gas stream.
It is known that SO2 can be converted into elemental sulphur with reducing agents and a number of processes have been proposed for this purpose. As a group, they are termed “SP-FGD” (sulphur-producing flue gas desulphurization). Some examples are the coal-based Foster Wheeler process (U.S. Pat. No. 4,147,762), the BaS/SO4-based cyclic process (M. Olper and M. Maccagni “Removal of SO2 from Flue Gas and Recovery of Elemental Sulfur” Euro. Pat. Appl. No. 728,698 Aug. 28, 1996), the Claus reaction-based McMaster-INCO process (U.S. Pat. No. 6,030,592) and the Na2S(aq)-based low-temperature process (Siu and Jia, 1999; Siu, 1999). To reduce SO2 to sulphur, CH4, H2 and CO gases are also used, often with a catalyst. A recently example is the CO-based process developed at MIT, in which a cerium oxide-containing catalyst is used (U.S. Pat. No. 5,242,673).